The present invention relates generally to wall-flow filters used to filter exhaust gases, and exhaust systems, as well as methods for making and incorporating and such filters.
Diesel exhaust systems may include, for example, a diesel particulate filters (DPFs) for removing particulates, such as soot from diesel exhaust. Where multiple DPFs are used to remove particulates, these DPFs are typically arranged in close proximity to each other and housed within a common enclosure, such as taught in US Pat. App. No. 2004/0161373. The most widely used DPFs are wall-flow filter. The conventional wall-flow filter consists of a ceramic honeycomb substrate having longitudinal, parallel cell channels formed by a plurality of intersecting porous walls. The ends of the cell channels are typically plugged with a ceramic plugging cement to form a checkered pattern of plugs at the end faces of the honeycomb substrate. The cell channels of the filter typically have some ends plugged at an inlet end face of the honeycomb substrate, referred to herein as “inlet channels.” Likewise, typically, the cell channels also have the remaining ends plugged to form a checkered pattern of plugs at an outlet end face of the honeycomb substrate, herein referred to as “outlet channels.” In use, exhaust gas containing entrained soot particles enters into the inlet channels, flows through the porous walls (i.e., the wall-flow) and into the outlet channels, and exits through the outlet channels, with the porous walls retaining a portion of the particles contained in the exhaust.
Conventional wall-flow filters may be cleaned out to prevent the filter from becoming blocked and to maintain a suitable pressure drop across the filter below a prescribed limit. Increase in pressure drop across the filter generally results in an increase in backpressure against the engine which, if not controlled, may lead to power loss. One known method for cleaning out the filter is to remove the soot trapped in the filter by thermal regeneration (hereinafter “regeneration”). The regeneration may be either “passive” or “active” or a combination thereof. In “passive” regeneration, the inlet temperature of the exhaust entering the filter is sufficiently high to itself initiate combustion of the soot trapped and additional energy input is required to raise the temperature of the exhaust (and the filter) to a level that would cause combustion of the soot trapped in the filter.
There are certain applications (e.g., off-road diesel engine applications and retrofit applications) where the specification on the wall-flow filter filtration efficiency is less rigorous, the wall-flow filters are required to have lower initial and total pressure drop, and smaller filter volume and low regeneration frequency are desirable. Conventional industry standard filters, as mentioned above, do not fulfill this need as a result of the filter's need to be regenerated on a regular/periodic basis and the resultant backpressure which builds up prior to the periodic regeneration.
Accordingly filters with reduced back pressures, moderately high clean and low filtration efficiency at high soot loading conditions, and which exhibit a soot bypass/self preservation feature at the same high soot loading levels/conditions thus requiring fewer regeneration events during operation, are desired for these off-road and retrofit applications.